The present invention relates generally to operations performed and equipment utilized in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides a reduced debris milled multilateral window.
In multilateral wells it is common practice to drill a branch or lateral wellbore extending laterally from an intersection with a main or parent wellbore. A casing string is typically installed in the parent wellbore, a whipstock is positioned in the casing string at the desired intersection, and then one or more mills are deflected laterally off of the whipstock to form a window through the casing sidewall.
Unfortunately, this milling process usually produces a large amount of debris, such as small pieces of the metal casing, which accumulate in the parent wellbore. This debris may make the whipstock difficult to retrieve after the milling process is completed. Even after the whipstock is retrieved, the debris may cause other problems, such as plugging flow control devices, damaging seals, obstructing seal bores, interfering with passage of equipment past the intersection, etc.
One proposed solution is to pre-mill the window in the casing, that is, form the window through the casing sidewall prior to installing the casing in the parent wellbore. However, if the casing is to be cemented in the main wellbore, the window should be closed during the cementing operation, such as by using an internal or external sleeve. Typically, the sleeve is made of an easily milled material, such as aluminum or a composite material, or is made so that it can be retrieved after the cementing operation.
Although such sleeves have achieved some success, they also have their problems. For example, the sleeve material may be incompatible with fluids used in the well. The use of an external sleeve increases the casing outer diameter, requiring either a smaller casing size to be used, or a larger wellbore to be drilled. The use of an internal sleeve reduces the casing inner diameter, restricting the passage of fluids and equipment through the casing. The use of a shiftable or retrievable inner sleeve requires another operation in the well and increases the complexity of the equipment and the procedure.
From the foregoing, it can be seen that it would be quite desirable to provide improved apparatus, systems and methods for forming windows in casing.